Killua Zoldyck is the third child of the infamous Zoldyck family, a lineage of legendary assassins residing on Kukuroo Mountain in the Republic of Padokea. Born as the middle son to Silva and Kikyo Zoldyck, he was identified from a young age as the most talented heir the family had ever produced, possessing potential that surpassed his siblings. His upbringing was devoid of a normal childhood; beginning at the age of three, he was subjected to relentless and brutal training designed to turn him into the perfect assassin, including exposure to lethal poisons, high-voltage electricity, and various forms of torture to build up resistances. By the age of twelve, despite his remarkable skills and status as the chosen successor, Killua grew deeply dissatisfied with a life defined solely by murder. This discontent culminated in him injuring his mother and his older brother Milluki before fleeing his home to take the Hunter Examination, claiming he wanted to do it for fun and to defy his family's expectations.

Killua’s personality is defined by a compelling duality, shaped by his violent upbringing and his desire for a different path. Initially, he projects the demeanor of a cheeky, cheerful, and mischievous boy, similar to his best friend but with a notably sharper tongue and a more cynical edge. He possesses a sharp, analytical mind, often assessing situations with remarkable speed and accuracy, and is known for his pragmatism and mature way of thinking, which contrasts with his youthful appearance. However, this lighthearted exterior masks the cold, efficient killer trained into him. When provoked, Killua can become deadly, violent, and bloodthirsty, reverting to his assassin instincts without hesitation. His core motivation shifts dramatically after leaving his family. While he initially sought to pass time and rebel, his primary drive becomes forging his own identity separate from the Zoldyck legacy. Meeting Gon Freecss provides him with his first real friendship, and protecting this bond and his new, self-chosen path becomes his central purpose.

In the narrative of Hunter × Hunter The Last Mission, Killua remains a central protagonist alongside Gon. While the film features original characters and conflicts involving the dark past of the Hunter Association, Killua's role aligns with his established character arc. He acts as a loyal and stalwart ally, utilizing his assassin skills and developing Nen abilities to combat the antagonistic forces of the movie. His primary motivation in this context is to support his friends and face the threats posed by those who wield Nen for vengeance, showcasing his growth from a solitary weapon into a young man who fights for others.

Key relationships are the primary catalyst for Killua’s development. His bond with Gon Freecss is the most significant, as Gon represents the first genuine friend Killua has ever had. This friendship challenges his ingrained assassin mentality, teaching him loyalty, compassion, and self-sacrifice. Killua often puts Gon’s safety and goals before his own, demonstrating an unwavering dedication that is both his greatest strength and, at times, a point of personal turmoil. Another pivotal relationship is with his older brother, Illumi, whose psychological manipulation and conditioning implanted a deep-seated fear and a needle in Killua’s brain that forced him to flee from any opponent perceived as stronger. Overcoming this influence is a crucial part of Killua's journey toward independence. His relationship with his sister Alluka later becomes paramount, showcasing his capacity for profound love and his willingness to defy his entire family to protect her.

Killua undergoes a profound development throughout the series, evolving from a fatalistic boy who sees himself only as a tool for killing into a young man with agency and a strong moral compass. His initial flaw is a compulsive need to retreat from any fight where victory is not guaranteed, a survival instinct implanted by his brother’s needle. Removing this needle is a transformative moment, freeing him from his paralyzing fear and allowing him to fight at his full potential. He learns to value his own life and to fight not just because he has to, but because he chooses to protect those he loves. This growth culminates in him finding a purpose beyond Gon, demonstrating a maturity and independence that solidifies his identity as a Hunter, not an assassin.

Notable abilities make Killua an exceptionally formidable fighter. He is a Transmutation type Nen user, meaning he can change the properties of his aura. His signature technique involves transmuting his aura into electricity, a feat made possible by his childhood torture training, which granted him a high tolerance to electric shocks. His abilities include Izutsushi or Lightning Palm, which delivers a paralyzing shock on contact, and Narukami or Thunderbolt, a powerful projectile of lightning. His most advanced technique is Kanmuru or Godspeed, which uses electricity to override his nervous system, allowing for automatic, reflexive movements far beyond his normal physical limits. This technique has two forms: Whirlwind, which enables his body to react to hostile intent without conscious thought, and Speed of Lightning, which allows for rapid, self-directed movement. Even without Nen, Killua possesses superhuman physical strength, able to open the Testing Gates of the Zoldyck estate, which weigh several tons, and transform his fingers into sharp claws. He also wields a pair of custom-made, heavy yo-yos as effective mid-range weapons.